A number of drugs with immunosuppressive actions have been known and they include alkylating agents, antimetabolites, antibiotics, steroids, folic acid antagonists and plant alkaloids.
Spergualin is a compound that was isolated by Umezawa, one of the inventors of the present invention, and others from the filtrate of a culture broth of Spergualin-producing microorganism of the genus Bacillus. The structural formula of Spergualin is shown below: ##STR4## Spergualin is not only effective against mouse leukemia L-1210, mouse leukemia EL-4, Ehrlich carcinoma and sarcoma 180 but also holds promise as an agent to control malignant tumors (U.S. Pat. No. 4,416,899).
Umezawa et al. continued their studies on Spergualin compounds and have synthesized numerous Spergualin compounds which are stabler and have a stronger anti-tumor activity (BP No. 2111480A, EP No. 105193A).
The immunosuppressive effects of steroids are considered to be accomplished principally through the anti-inflammatory action and the lysis of lymphocytes. However, as is well known, steroids have diversified pharmacologic effects and cause many side effects. The other immunosuppressants are classified as "cytotoxic" substances and their action is directed, among other things, to the pathways of nucleic acid synthesis and may often cause serious side effects on hematopoietic organs. It is therefore desired to develop immunosuppressive drugs that act selectively on lymphocytes and other cells of immunological importance while causing minimum side effects.